Arthrogryposis-renal dysfunction-cholestasis syndrome (ARC) is an autosomal recessive inherited disorder that leads to developmental defects in multiple organs systems. Symptoms include stiffness of joints, developmental defects in kidney and liver, scaly skin, agranular platelets, CNS anomalies and persistent infections and typically result in fatality within the first year after birth. ARC is caused by mutations in one of two genes, VPS33B or VPS16B/VIPAS39. The two ARC-linked proteins encoded by these genes are homologs of two of the six subunits of the HOPS complex, which is critical for the fusion of endosomes with lysosomes. This suggests that ARC is primarily a defect in membrane fusion events of endocytic pathways. To dissect the mechanisms that link these endocytic trafficking defects with their physiological consequences in ARC syndrome, Drosophila models of ARC were generated by mutating Vps33B and Vps16B. Surprisingly, both mutant lines were homozygous viable and fertile without any obvious developmental defects. However, both models of ARC syndrome revealed defects after infections: an inability to clear bacteria and severe hypersensitivity to infections despite an increased synthesis of anti-microbial peptides. This hypersensitivity of ARC models to infections was reversed by loss of function of the innate immune receptor PGRP-LC, indicating that the reason of lethality is an out-of-control host innate immune response. Importantly, the requirement of ARC proteins for the regulation of innate immune signaling is conserved in mammals, as knockdown of VPS33B caused equivalent defects in mammalian macrophages, including dramatically enhanced pro- inflammatory responses. Furthermore, we find that activated immune receptors in both fly tissues and mammalian macrophages get stranded in internal vesicles and possibly serve as the source for the elevated immune signaling we observe. Together, these findings raise the possibility that despite a primary defect in membrane trafficking, many of the symptoms of ARC patients may reflect exaggerated signaling. This proposal aims to investigate the mechanisms by which VPS33B-mediated endosomal maturation influences signaling and how, in turn, signaling alters endosomal trafficking and triggers shortened life span. Drosophila mutants eliminating different elements of the IMD pathway downstream of the PGRP-LC receptor will be tested in the context of Vps33B and Vps16B null alleles. Their effect on reduced life span, elevated immune signaling and altered endocytic trafficking in ARC mutants will be tested. These genetic experiments will be supplemented by pharmacological approaches blocking different kinases in the IMD pathway with the goal to suppress the hypersensitivity we observe in ARC models. Together, these experiments will provide a better understanding of the molecular mechanisms causing at least some of the symptoms in ARC syndrome and thus have the potential to significantly change the thinking about treatment options for ARC patients.